Vertical antennas must usually be operated above a ground plane which is either natural, man-made, or both, particularly where low frequency operation is desired. In such situations, the vertical antenna is either isolated from the earth by a base insulator or is connected directly to the earth and "shunt fed" by a "gamma match" or the equivalent thereof. In either situation, dipole radiation is accomplished through an "image" created by the ground plane at the base of the antenna. Typically, fifty or more conductive wires are buried in the earth a few inches below the surface thereof and extend radially outward from the base of the antenna for a distance of three-tenths of a wavelength or more in all directions. Thus, for low frequency transmission, installation of a ground radial system requires more "real estate" then is available to many who would like to benefit from the lower takeoff angles (less power) which a vertical antenna provides for long distance transmission. Further, the cost of installing such a ground radial system is substantial.
Antennas of varying effectiveness have previously been employed to circumvent the ground radial problem encountered with vertical antennas. Dipole antennas such as the well-known inverted "V" provide an essentially "horizontal" antenna with some vertical characteristics. Such antennas are unsightly and, while not requiring radials, take up a substantial amount of space above ground. The urban radio operator is thus still faced with problems of installation which cannot be reasonably solved by prior art antennas if adequate performance is to be achieved.
Antennas having a vertical orientation but without a ground radial system do exist. These antennas are "effectively" elevated above the ground, i.e., they radiate independently of the ground, and are associated with wavelengths of ten meters or less. Such antennas usually employ a pair of elements, i.e., dipoles, each a quarter wavelength long with the lower half of the dipole being electrically isolated from the vertical support structure. Such an arrangement requires structural isolation of the radiating portion of the vertical support structure, such as by a non-conductive structural section located between the ground and the radiating portion of the antenna.
According to the present invention, operation on wavelengths of 20 meters or greater can be accomplished by utilization of the metallic structure of the tower which is normally used simply to support other antennas. In effect, the metallic tower itself is caused to be an independent vertical antenna without the need for ground radial wires, without the need for insulating the tower itself from the ground, and without the need for base feeding the tower by the "shunt fed gamma match" system or its equivalent. Specifically, a conductive tower is isolated and caused to radiate from selected portions thereof so that radio signals can be transmitted or received in consideration of certain desirable conditions such as separation from RF absorptive or reflective objects, optimum electrical length, height above the ground, etc. The teachings of the present invention are also useful to restrict radiation from portions of the tower in order to prevent high RF absorption (power loss), RF radiation and excitation into other wires or circuits, and other undesirable effects which can be associated with the wide range of RF voltage and/or currents arising from the transmission of radio signals.
Although the invention is particularly useful for converting existing tower structures into vertical antennas for the lower frequency amateur radio bands the teachings of the invention are not restricted from use on any band or frequency. Considering the use of the invention in the amateur radio bands, a tower of the usual physical range of sizes can be used to support "yagi" or "quad" antenna for operating on the 10, 15, and 20 meter bands, and can also provide the capability for operating on the 40, 80, and 160 meter bands by combining the supporting tower and higher band antenna structures into a single element vertical antenna.
In order to effectively elevate a tower above ground so that an upper section can be excited, the upper portion of the tower must be isolated from the ground. According to the present invention, each conductive leg of a metal tower is fitted with a rectangular shaped inductor having a tunable condenser in the uppermost horizontal side of the inductor. The isolators which have a length which is effectively 1/4 of a wavelength, are physically attached to the legs of the tower in a manner such that a conductive path is formed between the isolators and the tower. The isolators act to effectively elevate the upper portion of the tower above ground without the need for physically forming the lower portion of the tower from a non-conductive, structurally different material.
A tower can be made to radiate (or to receive) by excitation caused by a rectangularly shaped conductive member, or exciter, which works against the side of the tower, the exciter being tuned by a tunable condenser. The effective length of the exciter is one quarter wavelength, the tunable condenser and a standoff insulator being disposed at the lower end thereof. An RF ground connection is located at the upper end of the exciter while an RF feed point for a coaxial transmission line is located near the grounded end of the exciter.
Accordingly, it is an object of the invention to provide a method and preferred apparatus for isolating from ground and exciting a conductive tower for use as a vertical antenna.
It is a further object of the invention to enable the use of a metal tower normally utilized for structural support as a vertical antenna which does not require a ground radial system.
Further objects and advantages of the invention will become more apparent in light of the following description of the preferred embodiments.